The present invention relates to a pneumatic radial tire, in which a reduction of the tire weight can be achieved, while maintaining the steering stability and the high speed running performance of the tire. This is achieved by preventing the tread portion from being cut and the tread groove bottoms from cracking caused during running.
Recently, from the point of view of preservation of the earth environment, energy-saving and the like, there is a great demand for vehicles whose fuel consumption is reduced. With respect to vehicle tires, it is necessary to reduce the tire weight.
Hitherto, in order to reduce the weight of a radial tire, it is known to make a tire belt from high elastic modulus organic fiber cords instead of steel cords. Aromatic polyamide fiber cords (hereinafter aramid cords) are typical thereof.
On the other hand, in a radial tire for high speed and heavy load use, in order to prevent the belt from being lifted during running, a so called jointless band belt (hereinafter JLB belt) is provided outside the breaker belt. The JLB belt is formed by winding a long narrow strip spirally and continuously in the tire circumferential direction.
In a tire having the above-mentioned construction, as the inner pressure increases, the bottoms of the tread grooves are subjected to a large stress, and tile groove bottoms are liable to crack during running. Therefore, the tire durability tends to decrease. When the tire is demolded, after vulcanization, the belt layer contracts radially to decrease its diameter, and the radius of curvature of the tread portion is greatly altered from the radius of curvature of the mold. If priority is given to the hoop effect for preventing lifting during running, it becomes hard to press the raw cover tire onto the mold impression during vulcanization, and as a result, the molding becomes inferior.
As a method for solving this problem, a JLB structure is disclosed in Japanese Patent application laid open No. 1-247204, in which a hybrid cord made of a high elastic modulus fiber strand (hereinafter high elastic modulus thread) and a low elastic modulus fiber strand (hereinafter low elastic modulus thread) is used. In this method, if non-metallic cords are used in the belt to reduce the weight, the problem of the tire contraction during vulcanization also arises. That is, as the belt has less pull-resistance, when the tire inner pressure is increased during vulcanization, the tire inflates and the band is elongated. As a result, the molding is superior. When the inner pressure is decreased after vulcanization, the cords return to almost the length as the original length while leaving a small permanent elongation (set). As the rubber accompanies such elongation and contraction, the diameter of the tire is increased when the tire is inflated with air. This means that the tread rubber is in an elongated state, in which the cut resistance of the tread rubber and crack resistance of the tread groove are decreased.
In Japanese patent application laid open No. 3-279004, an invention was disclosed in which a hybrid cord is used to decrease the shear between belt plies, while maintaining fitness to the vulcanizing mold. In this application, for example FIG. 2 shows that the band which is a belt reinforcing layer is not limited to only a JLB, and the stress at 6% elongation is 1 to 1.5 kgf.
Therefore, if the cord count is set at a usual level of about 50, the product of the stress and cord count is about 50 to 70 kgf.
In Japanese patent application laid open No. 4-169304, an invention was disclosed in which the quantity of the steel cords in a steel cord belt layer was properly reduced by using a JLB made of hybrid cords. However, there was no disclosure about the above-mentioned cut resistance and crack resistance.
In Japanese patent application laid open Nos. 4-8605 and 4-356205, a band made of a ribbon of rubberized parallel hybrid cords (hereinafter hybrid band) is disclosed. While there is no disclosure about the twist number for the hybrid cord, the stress at 3% elongation is described as being not more than 2.5 g/d. Further, it is explained that when less than 1 g/d, it is difficult to manufacture such a cord.
In British patent 2,064,445, the stress-elongation curves of various cords are shown in FIG. 3, wherein a 3.times.7 metallic cord displays a low stress of up to about 6% elongation, but when the elongation exceeds this value, the stress increases abruptly. In the disclosed invention, such a cord is used in a belt edge band, but two or more breaker belts made of metallic cords are provided. Therefore, it is not a structure intended to reduce the weight.
In a pneumatic radial tire, when non-metallic cords are used in its belt instead of steel cords to achieve a weight reduction while maintaining high speed durability and steering stability, the belt is required to be improved in the hoop effect to prevent lifting during high speed running, while displaying a proper elongation during vulcanization, which allows an easy inflation of the tire to fit the mold.
Even if both the requirements are satisfied, when elongation during vulcanization and hoop effect are such that the change in the tire diameter due to the margin of tire contraction is large, there is a problem that the tread portion is inferior in the cut resistance and crack resistance.